1. Timeline
Use your textbook and any other resources available to research the contributions and approximate dates of the following scientists/ models:
Democritus
John Dalton
JJ Thomson
Ernest Rutherford
Niels Bohr
Electron cloud model

2. Development of Atomic Theory

3. Democritus First proposed atomic theory in the 4th century BC
The universe is made of units called atoms (Greek for “unable to be cut or divided”)
Did not have enough evidence to convince others that his theory was true

4. Democritus

5. Dalton Atoms cannot be divided
1808
Revised atomic theory
Atoms cannot be divided
All atoms of a given element are exactly alike
Atoms of different elements join to form compounds

6. Dalton Experimental evidence Foundation of modern atomic theory
Law of definite proportions: a chemical compound always contains the same elements in the same proportions
Foundation of modern atomic theory

7. Law of Definite Proportions
Fill in the following table:
To calculate percent: divide mass of element by mass of compound.
Example: C=12 g, O=3 g
Percent C = 12/15 = .8 = 80%
Element 1
Element 2
Sample ID
Name
Percent
1001
1002
1003
1004
1005
1006

8. Thomson 1897 Cathode ray experiment
He found that cathode rays were made of negatively charged particles from inside atoms
This showed that atoms could be divided into smaller parts! (electrons)

9. Cathode-ray Tube Experiment

10. Plum Pudding Model
Electrons (negatively charged) are spread throughout the atom

11. Rutherford 1899 Atoms have a dense, positively charged nucleus
Developed his theory based on the gold-foil experiment
New model of the atom has electrons orbiting the positive nucleus

12. Gold-foil Experiment

13. “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

14. Bohr model 1913 Electrons only in certain energy levels (or shells)
Each level holds a certain number of electrons

15. Electron Cloud Model Developed in 1925 by Heisenberg and Schrodinger
Bohr’s model could not explain all electron behavior
Shows movement of electrons

16. James Chadwick
1932
Discovered the neutron by comparing proton mass to atomic mass